Tue, 13 Jul 2021, 01:00 pm - 02:00 pm
Gradute Thesis Defense Spring 2021
Bhupendra Man Shing Karki
University of Lousiville


Electron correlation effects in quantum materials are very strong. It is critical to investigate the structure of quantum materials to better understand and manipulate their physical properties. Quantum effects are prominent at the atomic microscopic length scale, which cannot be examined by average long range structural measurements using traditional diffraction methods. Instead, pair distribution function (PDF) analysis, a local structure probe, can effectively unveil the mystery of local structure, which is more sensitive to local behavior than bulk average features. The first section of my dissertation will concentrate on the local structural study of the Iron oxy-chalcogenides, La2O2Fe2OM2 (M = S, Se), which are layered materials formed by stacking layered units of La2O2 and Fe2OM2 (M = S, Se). Local crystal structure was studied using the pair distribution function technique, which involves Fourier transforming the measured total scattering intensity to obtain a real space representation of inter-atomic correlations. This technique was used to study local, short-range structural correlations that deviate from the average structure. Our results for M = S, Se show short-scale structural distortions in a typical range of 1-2 nm, indicating nematic fluctuations. However, neutron powder diffraction (NPD) provides clear evidence that the average, long-range structure remains tetragonal throughout the high and low temperature regimes. A comparable result was obtained for Fe1.1Te. This finding highlights the ubiquity of nematic fluctuations in iron-based superconductors and related materials.

The second part of my research is focused on measuring the transport and vibrational properties of black phosphorus and related materials. Phosphorene, a novel two-dimensional (2D) material, is gaining researchers' attention due to its exceptional properties, including a unique layer structure, a widely tunable band gap, strong in-plane anisotropy, and high carrier mobility. The effect of tensile strain on the Raman spectra of black phosphorus (BP) by using a simple custom strain device revealed clear red shifting of all three phonon modes, A1g, B2g and A2g. In a comparative study, we found that the effect of strain on the Raman shifting is larger for BP than that for MoTe2, presumably due to the smaller Young’s modulus of BP. We anticipate that our method of in-situ Raman spectroscopy could be an effective tool that can allow observation of strain effects directly, which is critical for future flexible electronic devices. In another study, temperature dependent transport properties of AsxP1-x (x = 0, 0.2, 0.5, 0.83, 1) alloys show that small arsenic doping greatly increases the thermoelectric power of black phosphorus. This alloy’s thermoelectric properties provide an environmentally friendly solution for direct and reversible conversion between heat and electricity. They have potential applications in a wide range of fields, including transportation, industry, and power generators/solid-state refrigerators, and may also provide solutions for sustainable energy sources.